GIANT HOGWEED
(Heracleum mantegazzianum)
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Giant hogweed can grow 15 to 20 feet
tall.
The stem and leaf stalks are hollow and
hairy with reddish-purple blotches. The
stem contains a clear watery sap.
The leaves are deeply cut and can be up
to five feet wide.
Giant hogweed flowers from mid-May
to July, with numerous white flowers
clustered in a flat, umbrella-shaped head
that is up to 2-1/2 feet in diameter.
The fruits are flattened, 3/8 inch long.
They split into two winged parts with
one seed in each.
Look-a-likes:
Cow parsnip (Heracleum
lanatum) is smaller, usually
growing to only three to
five feet, with flower heads
less than one foot in width.
Also, the leaves are not so
deeply incised.
Coltsfoot blooms in early
spring and is much smaller
than giant hogweed. It has
rounded leaves and
smaller, more rounded
flower heads.
Distribution:
There are at least thirty giant hogweed sites
scattered around Clallam County; many are
old homesites.
WHY BE CONCERNED?
Ø Hazardous to humans because skin contact
with the sap followed by exposure to
sunlight can cause painful burns and
permanent scars
Ø Crowds out desirable native plants which
supply food and habitat for wildlife.
Ø Damages riparian areas when it dies back in
the fall, leaving banks unvegetated and
susceptible to erosion.
Giant hogweed is a Class A Weed.
Control is required county-wide.
Ecology:
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Giant hogweed prefers rich, damp soil, but can spread along stream and roadsides.
Most plants die after they flower and produce seed, which can take four years or more. Individual
plants may produce additional crowns which continue to flower and set seed.
It spreads rapidly, mainly by seed. One flower head can produce thousands of seeds which are
distributed by wind, water, animals or people.
Giant hogweed can develop a massive root system that persists and is difficult to remove.
CONTROL
Prevention and early detection are the best means of control.
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Do not buy or plant giant hogweed.
Avoid introducing soil or gravel from areas known to have giant hogweed.
Remove seedlings when young because they can usually be pulled easily.
Replant the area with a desirable (preferably native) plant species, to discourage reinfestation.
Dispose of weeds properly, bag or burn seed heads.
Monitor the site for several years; promptly remove new seedlings.
HANDPULLING can control small infestations, but the entire root must be dug out or the plant may
grow back; care should be taken to avoid skin contact with the sap.
REPEATED MOWING during the growing season will weaken the plant and prevent it from
producing seed, but the roots remain alive for many years when plants are kept from flowering.
Known sites should be monitored and mowed as long as plants continue to appear. Avoid skin
contact with the sap that may ooze from mown fragments.
BIOLOGICAL CONTROL: Cattle and pigs will eat the plant without ill-effect. Trampling while
grazing also discourages growth.
HERBICIDES can be effective but should be applied with care. Read the label to check that you are
applying an herbicide in the right place, to the right plant, at the right time, and in the right amount. For
perennial weeds, long term control requires stopping seed production and attacking the weed’s root
system. Translocated herbicides, (ones that move throughout a plant’s system), such as Roundup
(glyphosate) and Crossbow (2,4-D + triclopyr) are recommended; testing shows that Roundup may
provide the best control.
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Apply to actively growing plants that are at least 18 inches tall.
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Note: Roundup is non-selective, and will kill target and non-target species alike.
Crossbow targets broadleaf plants without damaging grasses.
Warning: Giant hogweed often grows close to water and any herbicide application near or over water
requires a permit and a specially licensed applicator.
Call if you see this plant
Prepared by the Clallam County Noxious Weed Control Board Revised 11/2000
For more information call: 360-417-2442
or see www.clallam.net/weed